Field device electronics fed by an external electrical energy supply

ABSTRACT

An electric current controller for field-device electronics controls and/or modulates the supply current. The supply current is driven by a supply voltage provided by an external energy supply. The field-device electronics has an internal operating and evaluating circuit for controlling the field device, as well as an internal supply circuit, the internal input voltage being derived from the supply voltage for feeding the internal operating and evaluating circuit. In the supply circuit is a voltage controller flowed-through, at least at times, by a first component of the supply current. The voltage controller provides in the field-device electronics a first internal, useful voltage controlled to be essentially constant at a predetermined, first voltage level. Moreover, the supply circuit has a second voltage controller flowed through, at least at times, by a second current component of the supply current. The second voltage controller provides in the field-device electronics a second internal. Useful voltage, which is variable over a predetermined voltage range.

This is a Continuation of U.S. patent application Ser. No. 11/319,619filed on Dec. 29, 2005, which is the nonprovisional application of U.S.Provisional Application 60/639,791 filed on Dec. 29, 2004.

FIELD OF THE INVENTION

The invention relates to a field-device electronics for a field device.The field-device electronics is fed by an external electrical energy, orpower, supply. The invention relates, as well, to a field device havingsuch a field-device electronics.

BACKGROUND OF THE INVENTION

In the technology of industrial process measurements, especially also inconnection with the automation of chemical or technical-method processesand/or the control of industrial plants, measuring devices installednear to the process, so-called field devices, are used for locallyproducing measured-value signals as analog or digital representations ofprocess variables. Likewise, field devices can be embodied as adjustingdevices for varying one or more of such process variables and, in suchrespect, actively guiding the flow of the process. Such processvariables to be registered, or adjusted, as the case may be, include,for example, and as can also be perceived from the cited state of theart, mass flow rate, density, viscosity, fill level, limit level,pressure, temperature, or the like, of a liquid, powdered, vaporous orgaseous medium, conveyed, or stored, as the case may be, in acorresponding containment, such as e.g. a pipeline or a tank. Additionalexamples for such field devices, which are known, per se, to thoseskilled in the art, are described extensively and in detail in WO-A03/048874, WO-A 02/45045, WO-A 02/103327, WO-A 02/086426, WO-A 01/02816,WO-A 00/48157, WO-A 00/36379, WO-A 00/14485, WO-A 95/16897, WO-A88/02853, WO-A 88/02476, U.S. Pat. No. 6,799,476, U.S. Pat. No.6,776,053, U.S. Pat. No. 6,769,301, U.S. Pat. No. 6,577,989, U.S. Pat.No. 6,662,120, U.S. Pat. No. 6,574,515, U.S. Pat. No. 6,535,161, U.S.Pat. No. 6,512,358, U.S. Pat. No. 6,487,507, U.S. Pat. No. 6,480,131,U.S. Pat. No. 6,476,522, U.S. Pat. No. 6,397,683, U.S. Pat. No.6,352,000, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,285,094, U.S. Pat.No. 6,269,701, U.S. Pat. No. 6,236,322, U.S. Pat. No. 6,140,940, U.S.Pat. No. 6,014,100, U.S. Pat. No. 6,006,609, U.S. Pat. No. 5,959,372,U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,742,225, U.S. Pat. No.5,687,100, U.S. Pat. No. 5,672,975, U.S. Pat. No. 5,604,685, U.S. Pat.No. 5,535,243, U.S. Pat. No. 5,469,748, U.S. Pat. No. 5,416,723, U.S.Pat. No. 5,363,341, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,231,884,U.S. Pat. No. 5,207,101, U.S. Pat. No. 5,131,279, U.S. Pat. No.5,068,592, U.S. Pat. No. 5,065,152, U.S. Pat. No. 5,052,230, U.S. Pat.No. 4,926,340, U.S. Pat. No. 4,850,213, U.S. Pat. No. 4,768,384, U.S.Pat. No. 4,716,770, U.S. Pat. No. 4,656,353, U.S. Pat. No. 4,617,607,U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,574,328, U.S. Pat. No.4,524,610, U.S. Pat. No. 4,468,971, U.S. Pat. No. 4,317,116, U.S. Pat.No. 4,308,754, U.S. Pat. No. 3,878,725, EP-A 1158 289, EP-A1 147 463,EP-A 1 058 093, EP-A 984 248, EP-A 591 926, EP-A 525 920, or EP-A 415655, DE-A 44 12 388 or DE-A 39 34 007. The field devices disclosedtherein are, in each case, fed by an external, electrical energy supply,which provides a supply voltage and a supply current driven thereby,flowing through the electronics of the field devices.

For the case in which the field device serves as a measuring device, itadditionally contains an appropriate physical-to-electrical, orchemical-to-electrical, measurement transducer for electricallyregistering the relevant process variables. Such transducer is, mostoften, inserted in the wall of the containment carrying the medium orinto the course of a line, for instance a pipeline, conveying themedium, and serves to produce a measurement signal, especially anelectrical measurement signal, representing the primarily registeredprocess variable as accurately as possible. For processing themeasurement signal, the measurement transducer is, in turn, connectedwith the operating and evaluating circuit provided in the field-deviceelectronics and serving especially for a further processing orevaluation of the at least one measurement signal. In a large number ofsuch field devices, the measurement transducer is additionally soactuated by a driving signal generated, at least at times, by theoperating and evaluating circuit, that the transducer interacts at leastdirectly with the medium in a manner suitable for the measurement or,alternatively, essentially directly with the medium via an appropriateprobe, in order to provoke reactions reflecting the parameter to beregistered. The driving signal can, in such case, be controlled, forexample with respect to a current strength, a voltage level and/or afrequency. Examples of such active measurement transducers, thusmeasurement transducers appropriately converting an electric drivingsignal in the medium, are, especially, flow measurement transducersserving for the measurement of media flowing at least at times. Thetransducers utilize at least one coil actuated by the driving signal toproduce a magnetic field, or at least one ultrasound emitter actuated bythe driving signal, or a fill level, and/or limit level, transducerserving for measuring and/or monitoring fill levels in a container, suchas e.g. microwave antennas, Goubau lines, thus a waveguide for acousticor electromagnetic surface waves, vibrating immersion elements, or thelike.

For accommodating the field-device electronics, field devices of thedescribed kind further include an electronics housing, which, as e.g.disclosed in U.S. Pat. No. 6,397,683 or WO-A 00/36379, can be situatedremotely from the field device and connected therewith only via aflexible cable, or which, as shown e.g. also in EP-A 903 651 or EP-A 1008 836, is arranged directly on the measurement transducer or in, oron, a measurement transducer housing separately housing the measurementtransducer. Often, the electronics housing then serves, as shown, forexample, in EP-A 984 248, U.S. Pat. No. 4,594,584, U.S. Pat. No.4,716,770, or U.S. Pat. No. 6,352,000, also to accommodate somemechanical components of the measurement transducer, such as e.g.membrane, rod, shell or tubular, deforming or vibrating membersdeforming during operation, under the influence of mechanical forces;compare, in this connection, also the above-mentioned U.S. Pat. No.6,352,000. Field devices of the described kind are, furthermore, usuallyconnected together and/or with appropriate process control computers viaa data transmission system connected to the field-device electronics.The field devices transmit their measured value signals to suchlocations e.g. via (4 mA to 20 mA)-current loops and/or via digital databus and/or receive operating data and/or control commands incorresponding manner. Serving as data transmission systems here areespecially fieldbus systems, such as e.g. PROFIBUS-PA, FOUNDATIONfieldbus, as well as the corresponding transmission protocols. By meansof the process control computers, the transmitted measured value signalscan be processed further and visualized as corresponding measurementresults e.g. on monitors and/or converted into control signals for otherfield devices embodied as actuators, e.g. in the form ofsolenoid-controlled valves, electric motors, etc.

In the case of modern field devices, these are often so-called two-wirefield devices, thus field devices in the case of which the field-deviceelectronics is electrically connected with the external, electricalenergy supply solely via a single pair of electrical lines and in thecase of which the field-device electronics also transmits theinstantaneous measured value via the single pair of electrical lines toan evaluation unit provided in the external, electrical energy supplyand/or electrically coupled therewith. The field-device electronicsincludes, in such case, always an electrical current controller forsetting and/or modulating, especially clocking, such as strobing,triggering or firing, the supply current, an internal operating andevaluating circuit for controlling the field device, as well as aninternal supply circuit lying at an internal input voltage of thefield-device electronics derived from the supply voltage, feeding theinternal operating and evaluating circuit and having at least onevoltage controller, e.g. regulator, flowed through by a variable currentcomponent of the supply current and providing an internal useful voltagein the field-device electronics which is regulated, or controlled, to beessentially constant at a predeterminable voltage level. Examples ofsuch two-wire field devices, especially two-wire measuring devices ortwo-wire actuators can be found in, among others, WO-A 03/048874, WO-A02/45045, WO-A 02/103327, WO-A 00/48157, WO-A 00/26739, U.S. Pat. No.6,799,476, U.S. Pat. No. 6,577,989, U.S. Pat. No. 6,662,120, U.S. Pat.No. 6,574,515, U.S. Pat. No. 6,535,161, U.S. Pat. No. 6,512,358, U.S.Pat. No. 6,480,131, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,285,094,U.S. Pat. No. 6,269,701, U.S. Pat. No. 6,140,940, U.S. Pat. No.6,014,100, U.S. Pat. No. 5,959,372, U.S. Pat. No. 5,742,225, U.S. Pat.No. 5,672,975, U.S. Pat. No. 5,535,243, U.S. Pat. No. 5,416,723, U.S.Pat. No. 5,207,101, U.S. Pat. No. 5,068,592, U.S. Pat. No. 5,065,152,U.S. Pat. No. 4,926,340, U.S. Pat. No. 4,656,353, U.S. Pat. No.4,317,116, EP-A 1 147 841, EP-A 1 058 093, EP-A 591 926, EP-A 525 920,EP-A 415,655, DE-A 44 12 388, or DE-A 39 34 007.

For historical reasons, such two-wire field devices are, for the mostpart, so designed that a supply current instantaneously flowing in thesingle-pair line in the form of a current loop at an instantaneouscurrent strength set at a value lying between 4 mA and 20 mA, at thesame time, also represents the measured value produced by the fielddevice at that instant, or the actuating value instantaneously beingsent to the field device, as the case may be. As a result of this, aspecial problem of such two-wire field devices is that the electricpower at least nominally dissipatable or to be dissipated by thefield-device electronics—in the following referenced in short as“available power”—can fluctuate during operation in practicallyunpredictable manner over a wide range. To accommodate this, moderntwo-wire field devices (2L, or two line, field devices), especiallymodern two-wire measuring devices (2L measuring devices) with (4 mA to20 mA)-current loops, are, therefore, usually so designed that theirdevice functionality implemented by means of a microcomputer provided inthe evaluating and operating circuit is variable, and, to this extent,the operating and evaluating circuit, which, for the most part, does notdissipate much power anyway, can be adapted to the instantaneouslyavailable power.

A suitable adapting of the field-device electronics to the availablepower can e.g., as also proposed in U.S. Pat. No. 6,799,476, U.S. Pat.No. 6,512,358, or U.S. Pat. No. 5,416,723, be achieved by matching thepower instantaneously dissipated in the field device to theinstantaneously available power, and, indeed, in a manner such thatindividual functional units of the operating and evaluating circuit areoperated with appropriately variable clock speeds, or, depending on thelevel of the instantaneously available power, even turned off for aperiod of time (ready, or sleep, mode). In the case of field devicesembodied as two-wire measuring devices with active measurementtransducer, the electric power instantaneously dissipated in the fielddevice can, as disclosed in, among others, U.S. Pat. No. 6,799,476, U.S.Pat. No. 6,014,100, or WO-A 02/103327, additionally be matched to theinstantaneously available power by adapting also the electric powerinstantaneously dissipated in the measurement transducer, for example byclocking of the, as required, buffered driving signal, along with acorresponding matchable strobe rate, with which the driving signal isclocked, and/or by reducing a maximum current strength and/or a maximumvoltage level of the driving signal.

However, in the case of field devices embodied as two-wire measuringdevices, a varying of the device functionality has, for the most part,the result that, during operation, an accuracy, with which the operatingand evaluating circuit determines the measured value, and/or afrequency, with which the operating and evaluating circuit, for example,updates the measured value, are/is subject to changes in theinstantaneously available power. Also the buffering of excess powerpresent at times can only conditionally remedy this disadvantage oftwo-wire measuring devices with (4 mA to 20 mA)-current loops. On theone hand, due to the intrinsic explosion safety often required for suchtwo-wire measuring devices, at best, existing excess electrical energycan be stored in only very limited amounts internally in thefield-device electronics. On the other hand, however, the instantaneoussupply current, and, to such extent, also the, at best, available excessenergy, depends only on the instantaneous measured value, so that, inthe case of a lastingly very low, but, timewise, strongly varying,measured value, a correspondingly provided energy buffer can, over alonger period of time, become completely discharged. Moreover, forestablishing such a complex power management in the field device, a verycomprehensive and, thus, also very demanding power measurement isrequired, both with respect to circuitry and with respect to energy;compare, in this connection, also WO-A 00/26739, U.S. Pat. No.6,799,476, U.S. Pat. No. 6,512,358, or EP-A 1 174 841

Apart from this, it has been found, in the case of field devices of thedescribed kind having a measurement transducer for the conveying andmeasuring of media flowing at least at times, that the adaptive clockingof driving signals and/or of individual components of the operating andevaluating circuit is only conditionally suitable. This is true,especially in the application of a vibration-type measurementtransducer, such as described, for example, in the above-mentioned U.S.Pat. No. 6,799,476, U.S. Pat. No. 6,691,583, U.S. Pat. No. 6,006,609,U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,687,100, U.S. Pat. No.5,359,881, U.S. Pat. No. 4,768,384, U.S. Pat. No. 4,524,610, or WO-A02/103327. The field devices disclosed there serve to measure parametersof media flowing in pipelines, mainly mass flow rate, density orviscosity. To this end, the corresponding measurement transducer willinclude at least one measuring tube vibrating during operation andserving for the conveying of the medium, an exciter mechanismelectrically connected with the field-device electronics and having anoscillation exciter mechanically interacting with the measuring tube fordriving the measuring tube, as well as a sensor arrangement, whichgenerates measurement signals by means of at least one oscillationsensor arranged on the measuring tube, for locally representing themeasuring tube oscillations. Both the oscillation exciter and theoscillation sensor are, in such case, preferably of the electrodynamictype, thus constructed of a magnet coil and a plunging armatureinteracting therewith via a magnetic field.

Due to the highly accurate amplitude and frequency control of theexciter mechanism driving signal required for the operation of such ameasurement transducer, unavoidable, for one thing, is a timewisehigh-resolution sampling of the measuring tube oscillations. Equally, inthe case of measurements made on flowing media, the issued measuredvalue must also itself be updated often. On the other hand, a, mostoften, very high mechanical time constant of the oscillation systemformed by the measurement transducer leads to the fact that, in the caseof possible accelerations of the same, especially during non-stationary,transient happenings, a high driving power must be used and/orrelatively long settling times assessed. Further studies in thisconnection have, however, additionally shown that, because of theusually limited storage capacity for electric power, even a buffering ofexcess energy in the field device scarcely leads to any significantimprovement of the signal-to-noise ratio dependent on the amplitude ofthe measuring tube oscillations. In this respect, even a temporary andpartial switching-off of the operating and evaluating circuit is littlesuited for two-wire measuring devices with active measurement transducerof the described kind, especially for two-wire measuring devices havinga vibration-type measurement transducer involving the conveyance offlowing media.

A further possibility for improving the power capability of fielddevices of the described kind, especially two-wire measuring devices,is, at least in the case of minimal available power, to use as muchthereof as possible actually for the implementing of the devicefunctionality, thus to optimize a corresponding efficiency of the fielddevice, at least in the region of small available power. Supply circuitsfor the internal supply of the field electronics built on this principleare discussed in detail, for example, in U.S. Pat. No. 6,577,989, orU.S. Pat. No. 6,140,940. Especially, the solutions proposed therein aimto optimize the internally actually dissipatable, electrical power. Forthis purpose, there is provided at the input of the field-deviceelectronics, for adjusting and maintaining the above-mentioned, internalinput voltage of the field-device electronics at a predeterminable, asrequired also adjustable, voltage level, a voltage stabilizer, which, asa function of the instantaneously available power and an instantaneouslyactually needed power, has flowing through it, at least at times, avariable current component branched from the supply current. However, adisadvantage of this field-device electronics is that all internalconsumers are supplied practically from one and the same internal usefulvoltage and a possible collapse of this single useful voltage, forinstance because of too little supply current, can lead to a state inwhich normal operation of the field device is no longer possible, oreven to an abrupt, temporary, total stoppage of the field-deviceelectronics.

SUMMARY OF THE INVENTION

Starting from the above-discussed disadvantages of the state of the art,as viewed on the basis of the given examples of conventional2L-measuring devices, an object of the invention is to provide, for afield device of the described kind, a suitable field-device electronics,which makes it possible, at least in normal operation of the fielddevice, to keep the evaluating and operating circuit, especially amicroprocessor provided therein, continuously in operation and, in suchcase, to supply at least individual, selected, function units,especially the provided microprocessor, always with electric energy insufficient measure.

For achieving such object, the invention provides, for a field device, afield-device electronics fed from an external electrical energy supplyproviding an, especially unipolar, supply voltage and delivering, drivenby such voltage, an, especially unipolar and/or binary, variable supplycurrent, which field-device electronics includes:

-   -   a current controller, flowed-through by the supply current, for        adjusting and/or modulating, especially clocking, the supply        current,    -   an internal operating and evaluating circuit for controlling the        field device, as well as    -   an internal supply circuit lying at an internal input voltage of        the field-device electronics derived from the supply voltage,        and feeding the internal operating and evaluating circuit, the        internal supply circuit including    -   a first voltage controller flowed-through, at least at times, by        an, especially variable, first current component of the supply        current and providing in the field-device electronics an        internal, first useful voltage essentially constantly controlled        at a first, predeterminable, voltage level,    -   a second voltage controller flowed-through, at least at times,        by an, especially variable, second current component of the        supply current and providing in the field-device electronics an        internal, second useful voltage variable over a predeterminable        voltage range, as well as    -   a voltage stabilizer flowed-through, at least at times, by an,        especially variable, third current component of the supply        current and providing for the setting and maintaining of the        internal input voltage of the field-device electronics at a        predeterminable voltage level,    -   wherein the operating and evaluating circuit is flowed-through,        at least at times, both by an, especially variable, first useful        current driven by the first useful voltage, and by an,        especially variable, second useful current driven by the second        useful voltage.

Additionally, the invention resides in a field device including theaforementioned field-device electronics. In a first variant of the fileddevice of the invention, such serves for measuring and/or monitoring atleast one, predetermined, physical and/or chemical parameter, especiallya flow rate, density, viscosity, fill level, pressure, temperature,pH-value or the like, of a medium, especially a medium conveyed in apipeline and/or a container, and the field device includes therefor,additionally, a physical-electrical measurement transducer electricallycoupled with the field-device electronics, for reacting to changes ofthe at least one parameter and for issuing, at least at times, at leastone measurement signal corresponding with the parameter, especially avariable signal voltage and/or a variable signal current. In a secondvariant of the field device of the invention, such serves for theadjusting of at least one predetermined physical and/or chemicalparameter, especially a flow rate, a density, a viscosity, fill level,pressure, temperature, pH-value or the like, of a medium, especially amedium conveyed in a pipeline and/or container, and the field deviceincludes therefor, additionally, an electrical-to-physical actuatorelectrically coupled with the field-device electronics and reacting tochanges of at least one applied control signal, especially a variablesignal voltage and/or a variable signal current, with an adjustingmovement of the actuator for influencing the parameter to be adjusted.

In a first embodiment of the field-device electronics of the invention,the second useful voltage is controlled as a function of aninstantaneous voltage level of the internal input voltage of thefield-device electronics and/or as a function of an instantaneousvoltage level of a terminal voltage derived from the supply voltage anddropping initially across the field-device electronics.

In a second embodiment of the field-device electronics of the invention,the second useful voltage is controlled as a function of aninstantaneous current strength of at least one of the three currentcomponents. In a further development of this embodiment of theinvention, it is provided that the second useful voltage is controlledas a function of the instantaneous current strength of the third currentcomponent. In another further development of this embodiment of theinvention, it is provided further that the second useful voltage iscontrolled as a function of the instantaneous current strength of thesecond current component and an instantaneous voltage level of theinternal input voltage of the field-device electronics.

In a third embodiment of the field-device electronics of the invention,the feeding, external, energy supply provides a supply voltage having achanging, especially fluctuating, voltage level.

In a fourth embodiment of the field device of the invention, the supplyvoltage supplied from the external energy supply drives a supply currentof changing current strength, especially essentially a current strengthfluctuating in a manner undeterminable in advance.

In a fifth embodiment of the field-device electronics of the invention,a storage circuit is provided in the operating and evaluating circuit toserve for temporary storage of electrical energy.

In a sixth embodiment of the field-device electronics of the invention,the voltage stabilizer has components, especially a semiconductorelement or the like, serving primarily for the dissipation of electricalenergy and for the disposal of the heat energy arising therein.

In a seventh embodiment of the field-device electronics of theinvention, there is provided in the operating and evaluating circuit atleast one microprocessor and/or a digital signal processor, in which thefirst useful voltage, or a secondary voltage derived therefrom, serves,at least partially, as operating voltage.

In an eighth embodiment of the field-device electronics of theinvention, there is provided in the operating and evaluating circuit atleast one amplifier, in which at least one of the two useful voltages,or a secondary voltage derived therefrom, serves, at least partially, asoperating voltage.

In a ninth embodiment of the field-device electronics of the invention,there is provided in the operating and evaluating circuit at least oneA/D converter, in which the first useful voltage, or a secondary voltagederived therefrom, serves, at least partially, as operating voltage.

In a tenth embodiment of the field-device electronics of the invention,there is provided in the operating and evaluating circuit at least oneD/A converter, in which at least one of the two useful voltages, or asecondary voltage derived therefrom, serves, at least partially, asoperating voltage.

In an eleventh embodiment of the field-device electronics of theinvention, means are provided in the operating and evaluating circuitfor comparing electric voltages dropping in the field-device electronicsand/or electric currents flowing in the field-device electronics, withreference values. In a further development of this embodiment of theinvention, the operating and evaluating circuit produces an alarm signalsignalling an under-supplying of the field-device electronics, at leastwhen the operating and evaluating circuit detects a subceeding, orfalling beneath, by the second useful voltage, of a minimum usefulvoltage limit value predetermined for the second useful voltage and asubceeding, or falling beneath, by the third current component, of aminimum current component limit value predetermined for the thirdcomponent. In another further development of this embodiment of theinvention, the field-device electronics further includes at least onecomparator, which compares a sense voltage derived from the thirdcurrent component of the supply current with an associated referencevoltage and/or a comparator, which compares the second useful voltagewith at least one associated reference voltage.

In a twelfth embodiment of the field-device electronics of theinvention, such further includes sense resistors serving for producingsense voltages essentially proportional to current.

In a thirteenth embodiment of the field-device electronics of theinvention, such further includes a measuring and control unit forregistering and adjusting voltages dropping in the field-deviceelectronics, especially the second useful voltage, and/or currentsflowing in the field-device electronics, especially the second and/orthird current components. In a further development of this embodiment ofthe invention, the measuring and control unit controls the voltagestabilizer such that the third current component flows, when thecomparator comparing the second useful voltage with at least oneassociated reference voltage signals an exceeding by the second usefulvoltage of a maximum useful voltage limit value predetermined for thesecond useful voltage. In another further development of this embodimentof the invention, the measuring and control unit maintains a voltagedifference between the input voltage and the terminal voltage at apredetermined voltage level on the basis of the input voltage and/or theterminal voltage.

In a fourteenth embodiment of the field-device electronics of theinvention, the field-device electronics is electrically connected withthe external electrical energy supply solely via a single pair ofelectric lines.

In a first embodiment of the field device of the invention, suchcommunicates via a data transmission system, at least at times, with acontrol and review unit, with there being provided in the field-deviceelectronics for such purpose additionally a communication circuitcontrolling the communication via the data transmission system. In afurther development of this embodiment of the invention, the firstuseful voltage, or a secondary voltage derived therefrom, serves, atleast partially, as operating voltage for the communication circuit.

In a second embodiment of the field device according to the firstvariant, the operating and evaluating circuit of the field-deviceelectronics produces, at least at times, by means of the at least onemeasurement signal, a measured value representing instantaneously,especially digitally, the at least one parameter to be measured and/orto be monitored. In a further development of this embodiment of theinvention, the current controller adjusts the supply current on thebasis of the measured value instantaneously representing the at leastone parameter to be measured and/or monitored. In another furtherdevelopment of this embodiment of the invention, the supply current is achangeable direct-current, and the current controller is adapted tomodulate the measured value, at least at times, onto an amplitude of thesupply current.

In a third embodiment of the field device according to the firstvariant, the supply current is, at least at times, a clocked current,with the current controller being correspondingly adapted for clockingthe supply current.

In a fourth embodiment of the field device according to the firstvariant, the operating and evaluating circuit includes at least onedriver circuit for the measurement transducer, with the second usefulvoltage, or a secondary voltage derived therefrom, serving, at leastpartially, as operating voltage in the driver circuit. In a furtherdevelopment of this embodiment of the invention, the driver circuitcontains at least one operational amplifier. In another furtherdevelopment of this embodiment of the invention, the driver circuit hasat least one D/A converter and/or at least one signal generator forproducing the driver signal. According to a next further development ofthis embodiment of the invention, the measurement transducer has avariable, electrical impedance fed by the driver circuit, especially amagnet coil of variable inductance and/or a measuring capacitor ofvariable capacitance. Furthermore, it is provided that the electricalimpedance of the measurement transducer changes as a function of atleast one parameter to be measured and/or to be monitored. Additionally,it is provided that a signal voltage falling across the changingelectrical impedance and/or a signal current flowing through thechanging electrical impedance serves as measurement signal.

In a fifth embodiment of the field device according to the firstvariant, the operating and evaluating circuit has at least one A/Dconverter for the at least one measurement signal, in which the firstuseful voltage, or a secondary voltage derived therefrom, serves, atleast partially, as operating voltage. In a further development of thisembodiment of the invention, the operating and evaluating circuit has atleast one microcomputer connected with the A/D converter, especially amicrocomputer formed by means of a microprocessor and/or a signalprocessor, for generating the measured value, with the first usefulvoltage serving, at least partially, as an operating voltage of themicrocomputer.

In a sixth embodiment of the field device according to the firstvariant, the measurement transducer includes at least one measuring tubeinserted into the course of a pipeline for conveying the medium,especially a measuring tube vibrating, at least at times, duringoperation. In a further development of this embodiment of the invention,at least one magnet coil is arranged on the measurement transducer forproducing a magnetic field, especially a variable magnetic field. In anembodiment of this further development of the invention, the magnet coilhas, during operation of the measurement transducer, at least at times,an exciter current flowing through it, especially an exciter currentwhich is bipolar and/or variable in a current strength, for generatingthe magnetic field. Such exciter current is driven by the second usefulvoltage, or a secondary voltage derived therefrom. In another embodimentof this further development of the invention, the magnet coil interactsvia a magnetic field with a plunging armature, with magnetic field coiland armature being movable relative to one another. In anotherembodiment of this further development of the invention, the at leastone measuring tube of the measurement transducer vibrates, at least attimes, during operation, driven by an electromechanical, especiallyelectrodynamic, exciter mechanism formed by means of the magnetic fieldcoil and the plunging armature.

In a further development of the field device according to the firstvariant, the measurement transducer includes two measuring tubesinserted into the course of the pipeline for conveying the medium andvibrating, at least at times, during operation.

In a seventh embodiment of the field device according to the firstvariant, the measurement transducer serves for registering at least oneparameter, especially a fill level, of a container containing themedium, and includes therefor at least one measuring probe, especially amicrowave antenna, a Goubau line, a vibrating immersion element, or thelike, protruding into a lumen of the container or at least communicatingwith the lumen.

In an eighth embodiment of the field device according to the firstvariant, the field-device electronics is electrically connected with theexternal electrical energy supply solely via a single pair of electriclines and transmits the measured value, produced, at least at times, forrepresenting instantaneously, especially digitally, the at least oneparameter to be measured and/or monitored, via the single pair ofelectric lines to an evaluating circuit provided in the externalelectrical energy supply and/or electrically coupled therewith. In afurther development of this embodiment of the invention, aninstantaneous electrical current strength of the supply current,especially an instantaneous electrical current strength adjusted to avalue lying between 4 mA and 20 mA, represents the instantaneouslyproduced, measured value.

A basic idea of the invention is to divide consumers provided in thefield-device electronics—not counting the supply circuit itself—on theone hand, at least into a first group of electric circuits, orconsumers, of higher priority and into a second group of electriccircuits, or consumers, of lower priority, and, on the other hand, todesign the supply circuit so that, in normal operation of the fielddevice, at least the power, or energy, requirements of the first groupof electric circuits is always covered. Moreover, those circuits orcomponents, which mainly serve for storing energy internally in thefield device and/or cause electric energy to dissipate out of the fielddevice, can be assigned to a third group of electric consumers, whichhas current flow through it and thus is supplied with electric energysolely in the case of a sufficient supply of the first and secondgroups.

To the first group of electric circuits of higher priority areadvantageously assigned, among others, the at least one microprocessorprovided in the field-device electronics, along with the communicationcircuits serving for communication with possible, superordinated,control and review units. This has the advantage that the field devicecan, on the one hand, be kept permanently functioning and, on the otherhand, can also at least be kept permanently on-line. Furthermore, forthe case in which the field device is a measuring device, also themeasuring channel serving for the registering and conditioning of the atleast one measurement signal can primarily be assigned to the firstgroup of electric circuits, while possibly present, exciter channelsserving mainly for the operation of the electrical-to-physicalmeasurement transducer can be implemented as electric circuits of lowerpriority. This has, in the case of use of the field-device electronicsof the invention in a measuring device having a vibration-typemeasurement transducer, especially the advantage that practically theentire measuring channel extending from the oscillation sensors throughto the microprocessor can be operated with the essentially constantlycontrolled, first useful voltage and, therefore, can be suppliedpermanently in normal operation with the required electric power. Thishas the advantage that, to such extent, the measuring tube oscillationsproduced during operation can always be sampled at equally highfrequency and can also be processed with high resolution. Additionally,even though the exciter channel is operated partly or exclusively withthe variable, second useful voltage, the measuring tube can, in normaloperation, be excited essentially without any gaps, thus permanently,although, perhaps, with fluctuating oscillation amplitude. The inventionis based on, among other things, the discovery that neither temporaryshut-down of the microprocessor, nor intermittent operation of, forexample, the exciter channel can bring-about significant improvements inthe energy balance. Rather, it may important, on the one hand, topermanently supply the components with sufficient energy, which are ofvital importance for the operation of the field device and, ifapplicable, for communication with external devices. On the other hand,it may rather acceptable to supply components with an insufficientquantity of power, which are less essential for the operation of thefield device, or to shut down these components, if necessary.

Further, it has been found that it may more profitable to invest theavailable power prior in the at least one microprocessor and itsperiphery, particularly in processing and evaluating of the measurementsignal than in the exciter arrangement of the transducer, if available.Consequently the exciter arrangement of the transducer may supply withresidual of available power. Indeed, the optimal signal-to-noise ratiofor the measurement signal may not found in this manner each time. Butthis deficit in the quality of the measurement signal may compensatedwith the evaluating process running within the at least onemicroprocessor, which operates highly efficient yet. Particularly it hasbeen found that this concept may advantageous for field device, whichoperate continuously or at least quasi continuously, such as Coriolismass flow meters.

A further advantage of the invention is that the field device, becauseof the small power required for its operation, can, without more, meetthe specifications of the various explosion-protection classes. Thismakes the field device specially suited also for application in thoseexplosion-endangered areas, wherein only devices of intrinsic safety areallowed. Furthermore, the field device can, in such case, be soembodied, that it can work together with the usual field busses. Thiscan, on the one hand, occur by direct connection to the field bus, e.g.corresponding to the FIELDBUS-protocol (FIELDBUS is a registered mark ofthe FIELDBUS-FOUNDATION). On the other hand, the working together canoccur indirectly by means of a bus-coupler, e.g. corresponding to theso-called HART-protocol (HART is a registered mark of the HART UserGroup).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail on the basis ofexamples of embodiments, as well as on the basis of the drawing.Functionally equal parts are provided in the separate figures with thesame reference characters, which, however, are repeated in subsequentfigures only when such appears helpful. The figures of the drawing showas follows:

FIG. 1 perspectively in side view, a field device, as well as anexternal energy supply electrically connected therewith via a pair ofelectric lines;

FIG. 2 perspectively in a first side view, partially in section, anexample of an embodiment of a vibration-type measurement transducersuitable for the field device of FIG. 1;

FIG. 3 perspectively in a second side view, the measurement transducerof FIG. 2;

FIG. 4 an example of an embodiment of an electromechanicl excitermechanism for the measurement transducer of FIG. 2;

FIG. 5 in the form of a block diagram, a field-device electronicssuitable for application in a field device, especially a two-wire fielddevice;

FIGS. 6 to 8 partly in the form of block diagrams, circuits of examplesof embodiments of an exciter circuit suited for application in a fielddevice of FIG. 1 having a vibration-type measurement transducer of FIGS.2 to 4; and

FIGS. 9 to 11 circuit diagrams of examples of embodiments of end stagessuitable for the exciter circuits of FIGS. 6 to 8.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

While the invention is susceptible to various modifications andalternative forms, exemplary embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theintended claims.

FIG. 1 shows an example of a field device suited for application inindustrial measuring and automation technology, along with afield-device electronics 20 fed from an external, electrical energysupply 70. In operation, the external, electrical energy supply 70provides an, especially unipolar, supply voltage U_(V) and delivers inaccompaniment therewith a variable, especially binary, supply current Icorrespondingly driven by the supply voltage U_(V). For this purpose,the field-device electronics is electrically connected, duringoperation, with at least a pair of electric lines 2L. As a result of thevoltage drop naturally occurring between external energy supply 70 andthe input of the field-device electronics 20, the supply voltage U_(V)is, however, reduced over this distance to the terminal voltage U_(K)actually present at the input to the field-device electronics.

The field device serves, in an embodiment of the invention, formeasuring and/or monitoring, as well as for repeatedly delivering,measured values appropriately representing at least one, earlierdesignated, physical and/or chemical parameter, such as e.g. a flowrate, density, viscosity, fill level, pressure, temperature, pH-value,or the like, of a medium, especially a gas and/or a liquid, conveyed ina pipeline and/or a container. To this end, the field device includes,additionally, a physical-to-electrical measurement transducerelectrically coupled with the field-device electronics for reacting tochanges of the at least one parameter and for issuing, at least attimes, a measurement signal corresponding to the parameter, especiallyin the form of a variable signal voltage and/or a variable signalcurrent. Alternatively or supplementally, there can be provided in thefield device an electrical-to-physical actuator electrically coupledwith the field-device electronics for reacting to changes of at leastone applied control signal, especially in the form of a variable signalvoltage and/or a variable signal current, with the actuator providing anadjusting movement for influencing the parameter to be adjusted, or,stated differently, the field device can also, for example, be sodesigned that it serves for adjusting at least one of such physicaland/or chemical parameters of the medium. For controlling the fielddevice, especially also for activating the mentioned measurementtransducer or for activating the mentioned actuator, there is furtherprovided in the field-device electronics an internal operating andevaluating circuit 50. For the case in which the field device is ameasuring device serving for the measuring of the at least one, earlierdesignated, physical and/or chemical parameter, it is further providedthat the operating and evaluating circuit 50 determines the at least onemeasured value, or a plurality of corresponding measured values, for theparameter.

In the case of the field device illustrated in FIG. 1, such is anin-line measuring device serving especially for registering parameters,e.g. a mass flow rate, density and/or viscosity, of a medium, especiallya gas and/or a liquid, flowing in a pipeline (not shown), and forreflecting such in a measured value X_(M) instantaneously representingthis parameter. Accordingly, the field device can be, for example, aCoriolis mass flow measuring device, a density measuring device, or alsoa viscosity measuring device. For producing the at least one measurementsignal, the field device shown here includes a vibration-typemeasurement transducer 10 accommodated within a correspondingmeasurement transducer housing 100, as well as field-device electronics20 accommodated in the illustrated electronics housing 200 andelectrically connected in suitable manner with the measurementtransducer 10.

FIGS. 2 to 4 show an example of an embodiment for such a measurementtransducer, whose construction and manner of operation iscomprehensively described e.g. also in U.S. Pat. No. 6,006,609. It isnoted, however, already here, that, although the example of anembodiment of a field device in this instance concerns an in-linemeasuring device with a vibration-type measurement transducer, theinvention, of course, can be put into practice also in other fielddevices, for example in in-line measuring devices usingmagneto-inductive measurement transducers or acoustic measurementtransducers. Equally as well, the present invention can also be used infield devices which serve for measuring parameters, for example filllevel and/or limit level, such as are determined in connection withcontainers containing media. Such field devices are usually implementedby means of measurement transducers having at least one measurementprobe protruding into a lumen of the container or at least communicatingwith the lumen, for example a microwave antenna, a Goubau-line, avibrating, immersion element, or the like.

For conveying the medium to be measured, the measurement transducer 10of the example of an embodiment as shown in FIGS. 2 to 4 includes atleast one measuring tube 13, having an inlet end 11 and an outlet end12, a predeterminable measuring tube lumen 13A elastically deformableduring operation, and a predeterminable nominal diameter. Elasticdeformation of the measuring tube lumen 13A means, here, that, forproducing the above-mentioned, medium-internal, and, consequentlymedium-characterizing, reaction forces, a spatial shape and/or a spatialposition of the measuring tube lumen 13A is cyclically, especiallyperiodically, changed in predetermined manner within an elastic range ofthe measuring tube 13; compare e.g. U.S. Pat. No. 4,801,897, U.S. Pat.No. 5,648,616, U.S. Pat. No. 5,796,011 or U.S. Pat. No. 6,006,609. Incase required, the measuring tube can, as shown e.g. in EP-A 1 260 798,also be bent, for example. Moreover, it is e.g. also possible to use,instead of a single measuring tube, two bent or straight measuringtubes. Other suitable forms of embodiment for such vibration-typemeasurement transducers are described comprehensively e.g. in U.S. Pat.No. 6,711,958, U.S. Pat. No. 6,691,583, U.S. Pat. No. 6,666,098, U.S.Pat. No. 5,301,557, U.S. Pat. No. 5,357,811, U.S. Pat. No. 5,557,973,U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,648,616 or U.S. Pat. No.5,796,011. Especially suited as material for the straight measuring tube13 of FIGS. 3 and 4 are e.g. titanium alloys. Instead of titaniumalloys, however, also other materials usually used also for such,especially also bent, measuring tubes, can be used, such as e.g.stainless steel, tantalum or zirconium.

The measuring tube 13, which communicates in the usual manner at itsinlet and outlet ends with the pipeline conveying the medium into, andout of, the measuring tube, is held oscillatably in a rigid, especiallybending- and twisting-stiff, support frame 14 surrounded by themeasurement transducer housing 100. The support frame 14 is affixed tothe measuring tube 13 on the inlet end by means of an inlet plate 213and on the outlet end by means of an outlet plate 223, with these twoplates being, in each case, pierced by corresponding extension pieces131, 132 of the measuring tube 13. Furthermore, the support frame 14 hasa first side-plate 24 and a second side-plate 34, both of which plates24, 34 are affixed, in each case, in such a manner to the inlet plate213 and to the outlet plate 223, that they extend practically parallelto measuring tube 13 and are arranged spaced from this tube, as well asfrom each other; compare FIG. 3. Consequently, mutually facing sidesurfaces of the two side plates 24, 34 are likewise parallel to oneanother. A longitudinal strut 25 is fixed on the side plates 24, 34,spaced from the measuring tube 13, to serve as a balancing massabsorbing the oscillations of the measuring tube. The longitudinal strut25 extends, as shown in FIG. 4, essentially parallel to the entireoscillatable length of measuring tube 13; this is, however, notobligatory, since the longitudinal strut 25 can, of course, ifnecessary, also be made shorter. The support frame 14, with the two sideplates 24, 34, the inlet plate 213, the outlet plate 223 and thelongitudinal strut 25, thus has a longitudinal line of centers ofgravity extending essentially parallel to a measuring tube central axis13B virtually connecting the inlet end 11 and the outlet end 12. Theheads of the screws shown in FIGS. 3 and 4 are to indicate that thementioned securement of the side plates 24, 34 to the inlet plate 213,to the outlet plate 223 and to the longitudinal strut 25 can occur bythreaded connections; however, other suitable securement systems knownto those skilled in the are can be used as well. For the case in whichthe measurement transducer 10 is to be assembled releasably with thepipeline, the measuring tube 13 is provided with a first flange 119 onthe inlet end and a second flange 120 on the outlet end; compare FIG. 1.Instead of the flanges 19, 20, also other pipeline connecting pieces canbe provided for the releasable connection with the pipeline, such asindicated e.g. in FIG. 3 in the form of so-called triclamp connectors.In case required, the measuring tube 13 can also be connected directlywith the pipeline, e.g. by means of welding, hard-soldering or brazing,etc.

For producing the mentioned reaction forces in the medium, the measuringtube 13 is caused, during operation of the measurement transducer 10, tovibrate, and, thus, to elastically deform in predeterminable manner, ata predetermined oscillation frequency, especially a natural resonancefrequency, in the so-called wanted mode, driven by an electromechanicalexciter mechanism 16 coupled with the measuring tube. As alreadymentioned, this resonance frequency is also dependent on theinstantaneous density of the fluid. In the illustrated example of anembodiment, the vibrating measuring tube 13, as is usual for suchvibration-type measurement transducers, is spatially, especiallylaterally, deflected out of a static, rest position; the same isessentially true also for those measurement transducers, in which one ormore bent measuring tubes execute cantilever oscillations about acorresponding, imaginary, longitudinal axis virtually connecting the in-and out-let ends, or also for those measurement transducers, in whichone or more straight measuring tubes execute planar, bendingoscillations about a measuring tube longitudinal axis. In another case,in which the measurement transducer 10 executes, as described e.g. inthe mentioned WO-A 95/16 897, peristaltic, radial oscillations, so thatthe cross section of the vibrating measuring tube is, in the usualmanner therefor, symmetrically deformed, the longitudinal axis of themeasuring tube remains in its static, rest position.

The exciter mechanism 16 serves for producing an exciter force F_(exc)acting on the measuring tube 13, the exciter force being produced byconverting an electric exciter power P_(exc) fed from the operating andevaluating circuit 50 in the form of an electric driver signal. Theexciter power P_(exc) serves in the case of exciting a natural resonancefrequency essentially solely for compensation of the power fractionremoved from the oscillation system by mechanical and fluid-internalfriction. For achieving a highest possible efficiency, the exciter poweris, therefore, adjusted as accurately as possible, such that essentiallythe oscillations of the measuring tube 13 in the desired, wanted mode,e.g. in a fundamental resonance frequency, are maintained. For thepurpose of transferring the exciter force F_(exc) onto the measuringtube, the exciter mechanism 16 includes, as shown in FIG. 5, a rigid,electromagnetically and/or electrodynamically driven, lever arrangement15 having a cantilever 154 affixed rigidly on the measuring tube 13 andhaving a yoke 163. Yoke 163 is, likewise rigidly, affixed on one of theends of cantilever 154 spaced from the measuring tube 13, and, indeed,in such a manner that it is located above the measuring tube 13 andtransverse to it. Cantilever 154 can be e.g. a metal disk, or washer,which accommodates the measuring tube 13 in a bore. For other suitableembodiments of the lever arrangement 15, the already mentioned U.S. Pat.No. 6,006,609 is incorporated here by reference. Lever arrangement 15 isT-shaped and so arranged (compare FIG. 5) that it acts on the measuringtube 13 about at the half-way point, between inlet end 11 and outlet end12, whereby the measuring tube experiences, during operation, itsgreatest lateral deflection at such half-way point. For driving thelever arrangement 15, the exciter mechanism 16 of FIG. 5 includes afirst magnet coil 26 and an associated, first, permanently magnetic,armature 27, as well as a second magnet coil 36 and an associated,second, permanently magnetic, armature 37. The two magnet coils 26, 36,which are preferably connected in series, are affixed, especiallyreleasably, on both sides of the measuring tube 13, to the support frame14, beneath the yoke 163, such that they can interact with theirrespectively associated armatures 27, 37 during operation. The twomagnet coils 26, 36 can, if required, of course also be connected inparallel with one another. As shown in FIGS. 3 and 5, the two armatures27, 37 are affixed to yoke 163, mutually spaced from one another, insuch a manner that, during operation of the measurement transducer 10,armature 27 is essentially permeated by a magnetic field of coil 26 andarmature 37 essentially permeated by a magnetic field of coil 36, and onthe basis of corresponding electrodynamic and/or electromagnetic forces,they are moved, especially in a manner involving plunging in theirassociated magnet coils. The movements of the armatures 27, 37(especially in their functioning as plunging armatures) produced by themagnetic fields of the magnet coils 26, 36 are transferred by the yoke163 and by the cantilever 154 to the measuring tube 13. These movementsof the armatures 27, 37 are so developed relative to the respectivelyassociated magnet coils that the yoke 163 is deflected from its restposition alternately in the direction of the side plate 24 or in thedirection of the side plate 34. A corresponding axis of rotation,parallel to the already mentioned measuring tube central axis 13B canextend e.g. through the cantilever 154. The support frame 14 serving assupport element for the exciter mechanism 16 includes, additionally, aholder 29 connected, especially releasably, with the side plates 24, 34,for holding the magnet coils 26, 36, and, as required, individualcomponents of a magnet brake mechanism 217 discussed below.

In the case of the measurement transducer 10 of the example of anembodiment, the lateral deflections of the vibrating measuring tube 13held clamped securely at the inlet end 11 and the outlet end 12 effect,simultaneously, an elastic deformation of the lumen 13A of the measuringtube. This deformation develops over practically the entire length ofthe measuring tube 13. Furthermore, simultaneously to the lateraldeflections, twisting about the measuring tube central axis 13B iscaused in the measuring tube 13, due to the torque acting on such viathe lever arrangement 15, so that the measuring tube 13 oscillatesessentially in a mixed bending-torsional mode of oscillation serving aswanted mode. The twisting of the measuring tube 13 can, in such case, beso developed, that a lateral deflection of an end of the cantilever 154spaced from the measuring tube 13 is either equally, or oppositely,directed, compared to the lateral deflection of the measuring tube 13.The measuring tube 13 can, thus, execute torsional oscillations in afirst bending-torsional mode corresponding to the equally-directed caseor in a second bending-torsional mode corresponding to the oppositelydirected case. Then, in the case of the measurement transducer 10according to the example of an embodiment, the natural, fundamentalresonance frequency of the second bending-torsional mode of oscillationis approximately, at e.g. 900 Hz, twice as high as that of the firstbending-torsional mode. For the case in which the measuring tube 13 isto execute, during operation oscillations solely in the secondbending-torsional mode, a magnetic brake mechanism 217, operating on theeddy current principle, is integrated into the exciter mechanism 16, forstabilizing the position of the mentioned axis of rotation. The magneticbrake mechanism 217 can thus assure that the measuring tube 13 alwaysoscillates in the second bending-torsional mode and, consequently,possible external disturbances on the measuring tube 13 do not lead to aspontaneous switching into another bending-torsional mode, especiallynot into the mentioned, first mode. Details of such a magnetic brakingarrangement are described comprehensively in U.S. Pat. No. 6,006,609.

For causing the measuring tube 13 to vibrate, the exciter mechanism 16is fed during operation by means of a likewise oscillating excitercurrent i_(exc), especially one of adjustable amplitude and adjustableexciter frequency f_(exc), in such a manner that this current flowsthrough the magnet coils 26, 36 during operation and, in correspondingmanner, the magnetic fields required for moving the armatures 27, 37 areproduced. The exciter current i_(exc) is, as schematically shown in FIG.5, supplied from a driver unit 50B additionally provided in thefield-device electronics 20 and can be, for example, a harmonic,alternating current. The exciter frequency f_(exc) of the excitercurrent i_(exc) is, in the case of the example of an embodiment shownhere, preferably so selected, or it adjusts itself, such that thelaterally oscillating measuring tube 13 torsionally oscillates, to theextent possible, exclusively in the second bending-torsional oscillationmode.

It is to be noted here, in this connection, that, although in theexample of an embodiment shown here, the field-device electronics 20 hasonly one variable inductive impedance—in this case a magnet coil ofvariable inductance—fed by the driver unit 50B, the driver unit 50B canalso be designed to excite other electrical impedances, for example ameasuring capacitor of variable capacitance, or the like. In the case ofa capacitive pressure sensor as measurement transducer, its electricalimpedance would then change during operation also as a function of theat least one parameter to be measured and/or monitored, with, as isknown, a signal voltage falling across the changing electrical impedanceand/or a signal current flowing through the changing electricalimpedance serving as measurement signal.

For detecting the deformations of the measuring tube 13, the measurementtransducer 10 further includes a sensor arrangement, which, as shown inFIGS. 2 and 3, produces, by means of at least a first sensor element 17reacting to vibrations of the measuring tube 13, a first oscillationmeasurement signal for representing these vibrations and serving asmeasurement signal s₁. Sensor element 17 can be formed e.g. by means ofa permanently magnetic armature, which is affixed to the measurementtube and which interacts with a magnet coil held by the support frame14. Especially suited as sensor element 17 are especially those, which,based on the electrodynamic principle, register a velocity of thedeflection of the measuring tube 13. However, alsoacceleration-measuring, electrodynamic or even distance-measuring,resistive, or optical sensors can be used. Of course, also other sensorsknown to those skilled in the art and suitable for the detection of suchvibrations can be used, such as e.g. sensors registering strains of themeasuring tube 13. The sensor arrangement further includes a secondsensor element, especially one identical to the first sensor element 17,by means of which it delivers a second oscillation measurement signallikewise representing vibrations of the measurement tube 13 and, to suchextent, serving as a second measurement signal s₂. The two sensorelements 17, 18 are, in the measurement transducer illustrated in theexample of an embodiment, arranged mutually separated along themeasuring tube 13, especially at equal distances from the half-way pointalong the length of the measuring tube 13, such that the sensorarrangement 17, 18 locally registers both inlet- and outlet-endvibrations of the measuring tube 13 and presents them in the form ofcorresponding oscillation measurement signals.

FIG. 5 shows, schematically in the form of a block diagram, anembodiment of a field-device electronics 20 suitable for the fielddevice of FIGS. 1 to 4. On the right of FIG. 5, the above describedvibration-type measurement transducer is schematically illustrated, withexciter mechanism 16 and sensor arrangement 17, 18, with the magnetcoils required for the measurement principle of the transducer beingshown symbolically.

The first measurement signal s₁, and the second measurement signal s₂,which may, or may not, be present, both usually have a signal frequencycorresponding to the instantaneous oscillation frequency of themeasuring tube 13. These signals are, as shown in FIG. 2, fed to a,preferably digital, evaluation unit 50A of the operating and evaluatingcircuit provided in the field-device electronics 20. Evaluation unit 50Aserves for determining, especially numerically, a measured value, X_(M),instantaneously representing the process variable to be registered, heree.g. the mass flow rate, density, viscosity, etc., and to convert suchinto a corresponding measured-value signal xM available at the output ofthe operating and evaluating circuit. While, in the case of themeasurement transducer illustrated here, the density or also viscosityare readily determinable on the basis of just one of the measurementsignals s₁, S₂, for the determining of mass flow rate, both measurementsignals s₁, s₂ are used, in manner known to those skilled in the art,for ascertaining, for example in the signal time domain or in the signalfrequency domain, a phase difference corresponding with the mass flowrate.

In an embodiment of the invention, the evaluation unit 50A isimplemented using a microcomputer μC provided in the field-deviceelectronics 20. The microcomputer is so programmed that it digitallydetermines the measured value X_(M) on the basis of the measurementsignals delivered from the sensor arrangement 17, 18. For implementingthe microcomputer, e.g. suitable microprocessors and/or also modernsignal processors can be used. As also shown in FIG. 5, the evaluationunit 50A further includes at least one A/D converter, via which one ofthe sensor signals s₁, s₂ or, as usual especially in the case ofCoriolis mass flow transducers, a signal difference derived previouslyfrom the two sensor signals s₁, s₂, is supplied digitized to themicroprocessor. The measurement or operational data produced and/orreceived by the evaluation unit 50A can, furthermore, be storedvolatility and/or persistently in corresponding digital data memoriesRAM, EEPROM.

As already mentioned, the operating and evaluating circuit 50additionally contains a driver unit 50B for feeding the excitermechanism 16 with the mentioned exciter current i_(exc). Separateexamples of the driver unit 50B will now be explained on the basis ofFIGS. 6 to 11. As shown in FIG. 5, the driver unit 50B is also incommunication with the evaluation unit, especially the already mentionedmicroprocessor μC, from which the driver unit 50B receives e.g. therequired operating data, such as e.g. the instantaneously requiredexciter frequency or an amplitude instantaneously required for theexciter current, or to which the driver unit 50B sends internallyproduced adjustment signals and/or parameters, especially alsoinformation concerning the required exciter current i_(exc) and/or theexciter power P_(exc) fed into the measurement transducer. In additionto the microprocessor μC or instead of the same, the driver unit can,for example, also include a digital signal processor serving to producethe driver signal. FIG. 6 shows, in the form of a block diagram,examples of embodiments for the driver unit 50B, which are suitedespecially for use in a 2L measuring device. In a first variant, one ofthe sensor signals delivered from the sensors 17, 18 or e.g. also theirsum is fed to an amplitude demodulation stage pd as input signal. Thus,the amplitude demodulation stage pd is connected at its input with oneof the sensors 17, 18. In FIG. 6, that is the sensor 17. The amplitudedemodulation stage pd serves for determining continuously an oscillationamplitude of the measuring tube vibrations. Additionally, the amplitudedemodulation stage pd serves for delivering an output signal, e.g. asimple direct-current signal representing this registered oscillationamplitude. To this end, in a preferred embodiment of the invention, apeak value detector is provided for the input signal in the amplitudedemodulation stage pd. Instead of this peak value detector, also e.g. asynchronous rectifier can be used for registering the oscillationamplitude. The rectifier is clocked by a reference signal of equal phaseto the input signal. A first input of a comparison stage sa is connectedwith an output of the amplitude demodulation stage pd; a second input ofthe comparison stage sa receives an adjustable reference signal Sr,which specifies an amplitude of vibration of the measuring tube 13. Thecomparison stage sa determines a deviation of the output signal of theamplitude demodulation stage pd from the reference signal Sr and issuesthis as a corresponding output signal. This deviation can be determinedand forwarded on the basis of a simple difference between the registeredoscillation amplitude and that specified by the reference signal Sr inthe form of an absolute amplitude error or e.g. also on the basis of aquotient of registered and specified oscillation amplitudes in the formof a relative amplitude error. To a first input of an amplitudemodulation stage am1 is supplied the input signal of the amplitudedemodulation stage pd and, to a second input the output signal of thecomparison stage sa. The amplitude modulation stage am1 serves formodulating the input signal of the amplitude demodulation stage pd withthe output signal of the comparison stage sa. In such case, e.g. one ofthe sensor signals s₁, the sum of the two sensor signals s₁, s₂ or alsoa signal essentially proportional thereto, produced synthetically, forexample, by means of an appropriate signal generator, can serve as inputsignal, which, to such extent, is a carrier signal which can be quitevariable as to frequency. Onto this carrier signal is modulated theerror signal of variable amplitude, as produced by means of thecomparison stage sa. The error signal represents, namely, the deviationof the instantaneous vibration amplitude of the measuring tube 13 fromits, or their, desired oscillation amplitude represented by thereference signal Sr. Additionally, the amplitude modulation stage am1serves to deliver the driver signal carrying the driving energy for theexciter mechanism 16. For such purpose, the amplitude modulation stagehas a corresponding end stage ps for amplifying the carrier signalmodulated with the modulation signal. For the purpose of the amplitudemodulation of the carrier signal with the modulation signal, amultiplier m1 is additionally provided in the amplitude modulation stageam1; compare FIG. 6.

FIG. 7 shows, corresponding to the second variant of the invention,partly in the form of a block diagram, the circuit of a second variantfor the driver unit 50B. The example of an embodiment in FIG. 7 differsfrom that in FIG. 6 essentially in that, instead of its amplitudemodulation stage am1, a pulse-width modulation stage pwm is provided,having a pulse-length modulator pm clocked by an external alternatingcurrent signal. The pulse-length modulator pm is, as shown in FIG. 7,driven by a constant, positive, first, direct voltage +U1 and lies atcircuit ground, or zero point, SN. Supplied to a first input of thepulse-length modulator pm—that is the carrier signal input—is the inputsignal of the amplitude demodulation stage pd. Thus, this first input isconnected with one of the sensors—in FIG. 7 this is again the sensor 17.Supplied to a second input of the pulse-length modulator pm—this is themodulation signal input—is the error signal proportional to thedetermined amplitude error. The output of the pulse-length modulator pmis, in turn, connected with the input of an end stage ps′, which feeds,on its output side, the exciter mechanism 16 with a corresponding driversignal. The driver signal delivered from the end stage ps′ is, in thiscase, a rectangular signal, which is clocked with a signal frequency ofthe input signal of the amplitude demodulation stage pd and which has apulse width modulated with the output signal of the comparison stage sa.

FIG. 8 shows, partly in the form of a block diagram, the circuit of athird variant of the driver unit 50B. The example of an embodiment shownin FIG. 8 differs from that of FIG. 6 in that, instead of its multiplierm1, a comparator kk and a DC-DC converter dc are provided, whichdelivers at least one driver voltage driving the exciter currenti_(exc). The amplitude of this driver voltage is, in turn, dependent onthe output signal of the comparison stage sa and, therefore, is to beconsidered as non-constant. Depending on the driver voltage, the excitercurrent i_(exc) can, as already mentioned, be bi-polar or, however, alsounipolar. Consequently, the DC-DC converter dc delivers, in a preferredembodiment of the invention according to FIG. 8, a driver voltage havinga positive first potential +u and a negative second potential −u, with acontrol input of the DC-DC converter dc serving for adjusting of thepotentials and receiving the output signal of the comparison stage sa.The driver voltage delivered by the DC-DC converter dc, appropriatelyadapted in its amplitude, is applied to an end stage ps″ of the pulsewidth modulation stage pwm as operating voltage and the end stage ps″,in turn, feeds the exciter mechanism 16. Moreover, the end stage ps″ isconnected on its input side with an output of the comparator kk.Comparator kk is operated by the constant, positive, first directvoltage +U1 and lies at circuit ground SN. Supplied to an input of thecomparator kk is the input signal of the peak value detector pd.Consequently, comparator kk is connected on its input side with one ofthe sensors—in FIG. 8 this is again the sensor 17.

In FIGS. 6 to 7, it is indicated in each case by dashed lines that,instead of one of the sensor signals of the sensors 17, 18, also theirsum can be supplied to the peak value detector pd and to the multiplierm1, or to the pulse-length modulator pm, or to the comparator kk, as thecase may be; then, these sensor signals have to be passed through asumming unit. Alternatively, however, as already mentioned, a syntheticsignal can be used, produced by means of a digital signal processor anda D/A converter connected to its output, and correspondingly adapted tothe sensor signal in its frequency and phase position. In FIGS. 6 to 7,still other circuit portions are shown in dashed representation, toindicate preferred further developments of the preferred excitercircuit. In one further development of the driver unit 50B, apre-amplifier w is provided, which is placed in front of the peak-valuedetector pd or, as required, the synchronous rectifier. In anotherfurther development of the driver unit 50B, an amplifier v is provided,which amplifies the output signal of the comparison stage, before itreaches the amplitude modulation stage as error signal. Such anamplifier can be an operational amplifier op, whose non-inverting inputlies at circuit ground SN, whose inverting input is connected via aseries resistor wv with the output of the comparison stage sa and via ashunt resistor ws with the amplifier output. The operational amplifierconnected in this manner is, in each case, shown at the right top inFIGS. 6 to 7. In a next further development of the driver unit 50B, anintegrating amplifier vi is provided, which amplifies and integrates theoutput signal of the comparison stage sa, before it reaches themultiplier m as error signal. Such an amplifier can be an operationalamplifier op′, whose non-inverting input lies at circuit ground SN, andwhose inverting input is connected with the output of the comparisonstage sa via a series resistor wv′ and, via a series circuit formed of ashunt resistor ws′ and a capacitor k, with the output of the amplifier.The operational amplifier op′ connected in this manner is shown in eachcase in the right-middle of FIGS. 6 and 7.

Another further development of the driver unit 50B utilizes adifferentiating and integrating amplifier vd, which amplifies,differentiates and integrates the output signal of the comparison stagesa, before it reaches multiplier m1 as error signal. Such an amplifiercan be an operational amplifier op″, whose non-inverting input lies atcircuit ground SN, and whose inverting input is connected via a parallelcircuit of a series resistor wv″ and a first capacitor k1 with theoutput of the comparison stage sa and via a series circuit of a shuntresistor ws″ and a second capacitor k2 with the amplifier outpthismanner is shown in FIGS. 6 and 7 in each case at the right-bottom of thefigure. The arrows in FIGS. 6 and 7 indicate that the relevant amplifierv, vi, vd is to be placed in the box q (shown in dashed representation),which lies either between the output of the comparison stage sa and thesecond input of the amplitude modulation stage am, or, however, betweenthe output of the comparison stage sa and the modulation signal input ofthe pulse-width modulation stage pwm.

Quite within the framework of the invention is to have the functions ofthe individual circuit portions of FIGS. 6 and 7 implemented bycorresponding analog or digital circuit portions, in the latter case,thus e.g. by means of a suitable programmed microprocessor, with thesignals going to such being first passed through an analog/digitalconversion and its output signals, if required, being subjected to adigital/analog conversion.

FIG. 9 shows a circuit of a first example of an embodiment of an endstage ps, which can be inserted, for example, in the amplitudemodulation stage am of FIG. 6. An operational amplifier ov is powered bya positive and a negative, in each case constant, direct voltage +U, −Uand is connected as follows. An inverting input lies, via a firstresistor w1, at circuit ground SN and a non-inverting input is connectedvia a second resistor w2 to the output of the multiplier m1. An outputof the operational amplifier ov is connected through a third resistor w3with a first terminal pp1 of a primary winding of a transformer ff; asecond terminal pp2 of the primary winding lies at circuit ground SN.The secondary winding of transformer tf is connected by means of its twoterminals sp1, sp2 to the exciter mechanism 16.

The primary winding has a primary winding number N1 and the secondarywinding a secondary winding number N2. The transformer ff is a currentstep-up transformer and has a transformation ratio of e.g. 20:1. Theinverting input of the operational amplifier ov is connected through afourth resistor w4 to the first terminal pp1 of the primary winding. Thenon-inverting input is connected with the output through a fifthresistor w5. The five resistors w1, w2, w3, w4, w5 have correspondingresistance values R1, R2, R3, R4, R5. The resistance value R1 isselected equal to the resistance value R2, and the resistance value R4is selected equal to the resistance value R5. The alternating current iflowing into the exciter mechanism 16 is as follows, where um is theoutput voltage of the multiplier m: R5N1 1=um m R1 R3 N2.

FIG. 10 shows a circuit of a preferred, second example of an embodimentof an end stage ps′, which can be inserted, for example, in thepulse-width modulation stage pwm of FIG. 7. The “core” of thisembodiment of the end stage, which is a complementary push-pull endstage, is a series connection of the controlled current path of ap-channel-enhancement, insulating layer, field-effect transistor P withan n-channel-enhancement, insulating layer, field effect transistor N,which will be referenced in the following as “transistors” for short.The exciter mechanism 16 is connected to the junction point of thecontrolled current paths. On each controlled current path, a protectivediode dn, dp is connected in parallel, with each cathode lying on thepositive point of the associated transistor. The end of the seriesconnection on the p-transistor-side lies at a constant, positive, seconddirect voltage +U2 and its end on the n-transistor-side lies at acorresponding, negative direct voltage −U2. The gates of the transistorsN, P are connected together and with an output of the comparator kk′.The non-inverting input of the comparator kk′ lies on the output of thepulse-length modulator pm; compare FIG. 7. The inverting input of thecomparator kk′ is connected with a tap of a voltage divider composed ofa resistor r1 and a resistor r2. The resistors r1, r2 have the sameresistance values and lie between the positive, direct voltage +U1 andcircuit ground SN. The resistors r1, r2 and the comparator kk′ serve formaking the output signal of the pulse-length modulator pm symmetricalwith reference to the half-value of the direct voltage +U1. The excitermechanism 16 receives, consequently, at every positively directed edgethrough zero for the output signal of the sensor 17, or the sum of theoutput signals of the sensors 17, 18, as the case may be, a positivecurrent pulse and, at every negatively directed edge through zero forthe output signal of the sensor 17, or the sum of the output signals ofthe sensors 17, 18, as the case may be, a negative current pulse. Therespective durations of these current pulses are adjusted automatically,such that the oscillation amplitude of the measuring tube 13, asspecified by the reference signal Sr, is achieved.

FIG. 11 shows a circuit diagram of another example of an embodiment ofan end stage ps″, which can be inserted, for example, in the amplitudemodulation stage am1 of FIG. 8. The “core” of this embodiment of the endstage, which again is a complementary push-pull end stage, is, alsohere, as in the case of FIG. 10, a series connection of the controlledcurrent path of a p-channel-enhancement, insulating layer, field-effecttransistor P′ with an n-channel-enhancement, insulating layer, fieldeffect transistor N′, which will again be referenced in the following as“transistors” for short. The exciter mechanism 16 is connected to thejunction point of the controlled current paths. On each controlledcurrent path, a protective diode dn′, dp′ is connected in parallel, witheach cathode lying on the positive point of the associated transistor.The end of the series connection on the p-transistor-side lies at apositive, second direct voltage +u dependent on the output signal of thecomparison stage sa and its end on the n-transistor-side lies at anegative direct voltage −u dependent on the output signal of thecomparison stage sa. The gates of the transistors N′, P′ are connectedtogether and with an output of the comparator kk″. The non-invertinginput of the comparator kk″ lies on the output of the comparator kk;compare FIG. 8. The inverting input of the comparator kk″ is connectedwith a tap of a voltage divider composed of a resistor r3 and a resistorr4. The resistors r3, r4 have the same resistance values and lie betweenthe positive, direct voltage +U1 and circuit ground SN. The resistorsr3, r4 and the comparator kk″ serve for making the output signal of thecomparator kk symmetrical with reference to the half-value of the directvoltage +U1. The exciter mechanism 16 receives, consequently, at everypositive half-wave of the output signal of the sensor 17, or of the sumof the output signals of the sensors 17, 18, as the case may be, apositive current pulse and, at every negative half wave of the outputsignal of the sensor 17, or of the sum of the output signals of thesensors 17, 18, as the case may be, a negative current pulse. Therespective amplitudes of these current pulses are dependent on thedirect voltages +u, −u, which are themselves dependent on the outputsignal of the comparison stage sa, so that the oscillation amplitude ofthe measuring tube 13, as specified by the reference signal Sr, isachieved automatically.

The driver unit 50B, together with the measuring tube 13, represents acontrol loop, which electrically adjusts both to the mechanicalresonance frequency of the excited vibrations of the measuring tube 13and to the amplitude of these vibrations specified by means of thereference signal Sr. Therefore, the previously usual driver units,utilizing an amplitude control stage and a phase-locked loop, i.e. aso-called PLL, for electrical control of the resonance frequency and thevibration amplitude, are no longer required.

As already mentioned, the field-device electronics, and, to such extent,also the field device, are fed from an external, electrical energysupply 70, for example a remotely located measurement transmitter supplydevice or the like, which is connected with the field device, or, moreaccurately, with the field-device electronics 20, via the at least onepair of electric lines 2L. The measurement transmitter supply device, inturn, can, for example, be connected via a field bus system with asuperordinated process control system stationed in a process controlroom. In the example of an embodiment shown here, the field-deviceelectronics, as usual in a multitude of measurement and automationtechnology applications, is electrically connected with the externalelectrical energy supply solely via a single pair of electric lines 2L.Accordingly, the field-device electronics is thus, on the one hand,supplied with electric energy via this one pair of lines. On the otherhand, it is provided that the field-device electronics transmits themeasured value X_(M), produced at least at times, to an externalevaluating circuit 80 located in the external electric energy supply 70and/or electrically coupled with the energy supply, likewise via thesingle pair of electric lines 2L. The pair of electric lines 2L, in thiscase, the single pair, connecting the measurement transmitter supplydevice and the field device can, for example, for such purpose, beconnected in series with an energy source 71 feeding the supply currentI, e.g. a battery or a direct voltage source fed via aninstallation-internal supply network, and with measuring resistor R_(M).Energy source 70 drives the supply current I and supplies, therefore,the field-device electronics 20 with the electric energy required forits operation. The measuring resistor R_(M) is additionally providedwith two measuring terminals 72, 73, on which the supply currentinstantaneously representing the measured value X_(M) can be sensed inthe form of a current-proportional, measured voltage U_(M). The measuredvoltage U_(M) can be visualized on-site or fed to a superordinated,measured value processing unit. The—here, single—pair of electric lines2L can be embodied, for example, as a so-called two-wire, current loop,especially a 4 mA-20 mA current loop, or as a connecting line to anexternal, digital field bus, for example, a PROFIBUS-PA or aFOUNDATION-FIELDBUS.

In a further embodiment of the invention, it is, therefore, furtherprovided that the instantaneous measured value X_(M) is modulated ontothe supply current I. For example, the measured value instantaneouslydetermined by means of the field device can be represented by aninstantaneous current strength (especially a current strength adjustedto a value lying between 4 mA and 20 mA) of the supply current I flowingin the pair of electric lines 2L embodied as a two-wire current loop.

In another embodiment of the invention, it is provided that the fielddevice communicates, for example exchanges field-device-specific data,via a data transmission system, at least at times, with an externalcontrol and review unit, for example, a handheld operating device or aprogrammable logic controller. For this purpose, there is additionallyprovided in the field-device electronics 20 a communication circuit COM,which reviews and controls the communication via the data transmissionsystem. Especially, the communication circuit serves for converting,besides the measured value X_(M), e.g. also internal field-deviceparameters, into signals, which are transmittable over the pair ofelectric lines 2L, and for then coupling these signals into such lines.Alternatively or in supplementation thereof, the communication circuitCOM can, however, also be designed for correspondingly receivingfield-device parameters sent from the outside over the pair of electriclines 2L. The communication circuit COM can be, especially for theabove-described case in which the field device is connected duringoperation solely via a two-wire current loop to the external supplycircuit, e.g. an interfacing circuit working according to theHART@-Field-Communication-Protocol of the HART Communication Foundation,Austin, Tex., which uses FSK-coded, high-frequency, alternating voltagesas signal carrier.

As is evident from the combination of FIGS. 1 and 5, the field-deviceelectronics has, for the adjusting and control of voltages and/orcurrents internally in the field device, further, at least one currentcontroller IS₁, through which the supply current I flows, for adjustingand/or modulating, especially clocking, of the supply current I.Additionally provided in the field-device electronics 20 is an internalsupply circuit 40, which lies at an internal input voltage U_(e) of thefield-device electronics 20 derived from the terminal voltage U_(K) andwhich serves for the electrical feeding of the internal operating andevaluating circuit 50.

For registering and regulating voltages instantaneously dropping in thefield-device electronics 20 and/or instantaneously flowing currents, thesupply circuit further includes a corresponding measuring and controlunit 60. Moreover, the measuring and control unit 60 serves, especiallyfor the above-mentioned case in which the measured value X_(M) ismodulated onto the supply current I, also for converting ameasured-value signal X_(M), as supplied from the operating andevaluating circuit 50 and representing the instantaneously produced,measured value X_(M), into a correspondingly controlling, first currentcontrol signal I_(control) controlling the current controller and, tosuch extent, also the supply current. The current control signalI_(control) is, in an embodiment of the invention, so adapted that thecurrent controller IS₁, becomes able to control the supply current I onthe basis of the instantaneously determined measured value X_(M)proportionally thereto. Alternatively, or in supplementation thereof,the current control signal I_(control) is so developed that the currentcontroller IS₁ strobes the supply current, for example binary coded forthe purpose of communication according to the standard PROFIBUS-PA. Forproducing correspondingly current-representing, especially essentiallycurrent-proportional, sense voltages I₁ _(—) _(actual), I₂ _(—)_(actual), I₃ _(—) _(actual), corresponding sense resistors R₁, R₂, R₃are additionally provided in the supply circuit 40. At least at times,the supply current, or current components I₁, I₂, I₃ derived therefrom,flow through the respective resistors R₁, R₂, R₃.

At least for the above-described case, in which the supply current ismodulated in its amplitude for the purpose of representing the measuredvalue X_(M), and, due to the limited electric power of the externalenergy supply, the supply voltage U_(V) delivered therefrom and,consequently, associated therewith, also the terminal voltage U_(K)correspondingly sink with increasing supply current I, or, the reverse,with sinking supply current I they again increase, the supply-voltageU_(V) and, to such extent, also the terminal voltage U_(K) are to beconsidered fluctuating in voltage level in, at first, non-determinablemanner and, to such extent, variable during operation in significantmeasure. When the field device works according to the above-mentioned,in industrial measurement technology long-established standard of 4 mAto 20 mA, the only available current range for energy supply in normaloperation is that beneath 4 mA, and, depending on the level of thesupply voltage, the permanently available electric power is then onlyaround 40 to 90 mW.

The supply circuit 40 therefore has, as also schematically shown in FIG.5, additionally, at the input, a voltage stabilizer 30, which isprovided for the purpose of adjusting an internal input voltage U_(e)(serving as primary, or base, voltage for the internal energy supply) ofthe field-device electronics as accurately as possible at apredetermined voltage level and for maintaining such at this voltagelevel also as constantly at the same level as possible, at least for theundisturbed, normal operation. For the further, internal, distributionof the electric energy to individual components or groups of thefield-device electronics, such further includes, for converting thestabilized, internal, input voltage U_(e), a first voltage controllerUR₁, which, at least at times, is flowed-through by an, especiallyvariable, first current component I₁ of the supply current and whichserves for providing a first internal, useful voltage U_(N1) in thefield-device electronics. This voltage U_(N1) is essentially constantlycontrolled to a predetermined, as required also parameterable, desiredfirst voltage level U_(N1) _(—) _(desired). Additionally provided in thesupply circuit 40 is a second voltage controller UR₂ likewise convertingthe stabilized, internal input voltage U_(e). This second voltagecontroller UR₂ is flowed-through, at least at times, by an especiallyvariable, second current component I₂ of the supply current I. Thesecond voltage controller UR₂, in turn, serves for making available inthe field-device electronics 20 a second internal useful voltage U_(N2),which is variable over a predetermined voltage range. The voltage levelfor the useful voltage U_(N2) best-suited for the instantaneoussituation as regards consumption in the field-device electronics can bedetermined, for example, by the measuring and control unit 60 withregard to an instantaneous consumption situation in the field-deviceelectronics and then forwarded correspondingly to the voltage controllerUR₂ in the form of a voltage control signal U_(N2) _(—) _(desired).Voltage controllers UR₁, UR₂ can be, for example, so-called switchingcontrollers, or regulators, and/or unclocked linear controllers, orregulators, while the voltage stabilizer can be formed, for example, bymeans of a shunt-regulator IS₂ lying in a bypass to the internal inputvoltage U_(e), for example one implemented by means of a transistorand/or an adjustable Zener diode.

Beyond this, the voltage stabilizer 30 is, as also shown in FIG. 5, sodesigned that a third, especially variable, current component I₃ of thesupply current I flows through it, at least at times, during normaloperation, with the measuring and control unit 60 delivering a secondcurrent control signal I₃ _(—) _(control) appropriately controlling thevoltage stabilizer 30—here the shunt regulator IS₂—and, to such extent,also determining the third current component. The current control signalI₃ _(—) _(control), in such instance, so designed, at least for the casein which the electrical power instantaneously available in thefield-device electronics 20, resulting from internal input voltageU_(e), which is maintained essentially constant, and from theinstantaneously set supply current I, exceeds the electrical poweractually instantaneously needed on the part of the operating andevaluating circuit 50, that it causes a transistor provided in thevoltage stabilizer 30 to become conductive to a sufficient degree that asufficiently high current component I₃ is caused to flow for thestabilization of the input voltage U_(e). For this purpose, the voltagestabilizer 30 has, in a further embodiment of the invention, alsocomponents, especially a semiconductor element with cooling fin, or thelike, serving for the dissipation of electric energy and for getting ridof heat energy associated therewith. On the other hand, however, thecurrent control signal I₃ _(—) _(control) is also so designed that, inthe case in which the need for power in the operating and evaluatingcircuit 50 becomes greater, it again lessens the current component I₃instantaneously flowing in the voltage stabilizer 30.

As also shown in FIG. 5, it is further provided in the field-deviceelectronics 20 of the invention, and, to such extent, also in the fielddevice of the invention, that the operating and evaluating circuit 50 isflowed-through, at least at times, both by a first useful currentI_(N1), especially such a current which is variable, driven by the firstuseful voltage U_(N1), which is kept essentially constant, at least innormal operation, and by a second useful current I_(N2), especially sucha current which is variable, driven by the second useful voltage U_(N2),which is allowed to vary during operation. This has the advantage thatat least the assemblies and circuits of the field-device electronics 20,especially the mentioned at least one microprocessor μC, controlling thefield device during normal operation and, to such extend, keeping thefield device operational, can always be supplied with the electricalenergy that they actually instantaneously need. Accordingly, it isprovided in an embodiment of the invention, that the above-mentionedmicroprocessor μC and/or the mentioned signal processor are/is operated,at least partially, with first useful voltage U_(N1) largely heldconstant during normal operation, or with a secondary voltage derivedtherefrom. In a further development of this embodiment of the invention,the first useful voltage U_(N1), or a secondary voltage derivedtherefrom, serves further, at least partially, also as operating voltagefor the at least one A/D-converter provided in the operating andevaluating circuit. In a further embodiment of the invention, it isprovided that at least also the components of the field-deviceelectronics controlling and maintaining the communication with thementioned, superordinated control and review unit, thus, here, besidesthe microprocessor μC, also the communication circuit COM, are, at leastpartially, supplied by means of the first useful voltage U_(N1) or by asecondary voltage derived therefrom.

Depending on which power can actually be made available during operationon the part of the external supply circuit 70 and as a function also ofthe actual power requirement of the consumers fed already, in theabove-described manner, by the first useful voltage U_(N1), alsoindividual components of the driver unit 50B, especially such whichserve for producing the driver signal i_(exc), for example amplifiers,D/A-converters and/or signal generators, etc., provided therein, can,additionally, be fed, at least partially, by means of the first usefulvoltage U_(N1) or a secondary voltage derived therefrom. However, it hasbeen found that, alone already with currently obtainable microprocessorsμC and/or A/D-converters and the peripheral circuits required therefor,one must already reckon with a permanent power requirement of about 30mW in normal operation, so that, at least in the case of applicationshaving a permanently available power of only about 40 mW, thus withterminal voltages of 12 V or less, the aforementioned components of thedriver unit 50B can only still be connected to the first useful voltageU_(N1) to a very limited extent, without endangering the desired, highstability. To such extent, an embodiment of the invention furtherprovides that individual components of the driver unit 50B are operated,especially for longer periods of time, only using the second usefulvoltage U_(N2). Especially, the second useful voltage U_(N2) is suitableas operating voltage for the operational amplifier provided in thedriver unit 50B. Accordingly then, the exciter current i_(exc) for themagnetic field coils are driven essentially by the second useful voltageU_(N2) or a secondary voltage derived therefrom.

For bridging-over transient voltage fluctuations on the part of thesupply voltage and/or for buffering possible short-time “overloadings”of the internal field-device voltage supply due to a momentarily higherinternal power requirement, for example in the case of start-up of themeasurement transducer or during writing of the mentioned, persistentmemory EEPROM, a further development of the invention provides in theoperating and evaluating circuit a storage circuit, especially acapacitive storage circuit, serving for the temporary storage ofelectric energy. The energy buffer C is shown as part of the voltagestabilizer in the example of an embodiment illustrated here, so that itlies essentially permanently at the internal input voltage U_(e).However, in order to be able to prevent, safely, a collapse of theuseful voltage U_(N1), at least in normal operation, it is, of course,important to make certain, at the beginning, in the design of theassemblies and circuits supplied by means of the first useful voltageU_(N1), that their maximum consumed electrical power is, at most, equalto a minimum available electric power in normal operation and/or itsinstantaneously consumed electric power is at most equal to aninstantaneously available power.

In a further embodiment of the invention, it is provided, additionally,that the second useful voltage U_(N2) is controlled during operation asa function of an instantaneous voltage level of the internal inputvoltage U_(e) of the field-device electronics. Alternatively or insupplementation thereto, it is provided that the second useful voltageU_(N2) is controlled as a function of an instantaneous voltage level ofa terminal voltage U_(K) derived from the supply voltage and fallingfrom the input, across the field-device electronics. It has,furthermore, been found to be advantageous, in this connection, tocontrol the internal input voltage U_(e) such that a voltage differenceexisting between this and the terminal voltage U_(K) is held as constantas possible, for example at about 1 V, at least during normal operation.This makes it possible, among other things, to adjust the input voltageU_(e) relatively accurately, even in the case of changing operatingtemperature of the current controller IS₁ and a change of its transfercharacteristic associated therewith and so, in simple manner, to achievea very robust control of the internal input voltage U_(e). The controlcan, in such case, be implemented, for example, by means of a differenceamplifier provided in the mentioned measuring and control unit 60. Thedifference amplifier subtracts a sense voltage correspondingly derivedfrom the internal input voltage U_(e) from a sense voltagecorrespondingly derived from the terminal voltage U_(K). Alternativelyor in supplementation thereto, the second useful voltage U_(N2) can alsobe controlled as a function of an instantaneous current strength of atleast one of the three current components I₁, I₂, I₃. For example, thesecond useful U_(N2) can be controlled as a function of theinstantaneous electrical current strength of the third current componentI₃, which, taking into consideration the instantaneous input voltageU_(e), essentially represents an excess power instantaneously present inthe field-device electronics. Suitable as measured quantity, in thiscase, is especially also the second current control signal I₃ _(—)_(control) controlling the voltage stabilizer and, to such extent, alsodetermining the third current component I₃.

For determining and/or monitoring an instantaneous operating state ofthe field-device electronics, a further development of the inventionadditionally provides means for comparing electric voltages falling inthe field-device electronics and/or electric currents flowing in thefield-device electronics with predetermined, especially adjustable,threshold values. Such means for comparing voltages and/or currents can,for example, be embodied as integral component(s) of the alreadymentioned, measuring and control unit of the supply circuit. In anembodiment of this further development of the invention, the means forcomparing are so designed that, on the part of the field-deviceelectronics, an alarm signal x_(pwr) _(—) _(fail) signaling anunder-supplying of the field-device electronics is produced, at leastwhen a subceeding, or falling beneath, of a minimum useful voltage limitvalue, predetermined for the second useful voltage U_(N2), by the seconduseful voltage U_(N2) and a subceeding of a minimum current componentlimit value, predetermined for the third current component I₃, by thethird current component I₃ are detected. Serving for registering thethird current component I₃ can be e.g. a sense-resistor R3 provided inthe voltage stabilizer and correspondingly flowed-through by the currentcomponent I₃, to yield an essentially current-proportionalsense-voltage. In a further embodiment of the invention, the measuringand control unit controls the voltage stabilizer by means of the currentcontrol signal I₃ _(—) _(control), such that the third current componentI₃ flows, especially only when the comparator comparing the seconduseful voltage with at least one associated reference voltage signals anexceeding by the second useful voltage of a maximum useful voltage limitvalue predetermined for the second useful voltage. The means forcomparing voltages and/or currents can be, for example, simplecomparators, which compare, in each case, the sense voltage with anassociated reference voltage, internally produced, for example, by meansof the input voltage U_(e) and being, in each case, proportional to thethreshold value.

While the invention has been illustrated and described in detail in thedrawings and forgoing description, such illustration and description isto be considered as exemplary not restrictive in character, it beingunderstood that only exemplary embodiments have been shown and describedand that all changes and modifications that come within the spirit andscope of the invention as described herein are desired to protected.

1. Field-device electronics for a field device, said electronics beingfed by an external, electric energy supply providing a supply voltageand delivering a variable supply current driven by said supply voltage,said field-device electronics comprising: an electric current controllerfor adjusting and/or modulating said supply current, said currentcontroller being flowed-through by said supply current; an internaloperating and evaluating circuit for controlling the field device; andan internal supply circuit feeding said internal operating andevaluating circuit, said internal supply lying at an internal inputvoltage of the field-device electronics derived from the supply voltage,and said internal supply circuit including a first voltage controllerflowed-through, at least at times, by a first current component of thesupply current for providing in the field-device electronics a firstinternal, useful voltage, a second voltage controller flowed-through, atleast at times, by a second current component of the supply current forproviding in the field-device electronics a second internal, usefulvoltage variable over a predeterminable voltage range, and a voltagestabilizer flowed-through, at least at times, by a third currentcomponent of the supply current for adjusting and maintaining saidinternal input voltage of the field-device electronics at apredeterminable voltage level; wherein the operating and evaluatingcircuit is flowed-through, at least at times, both by a first usefulcurrent driven by the first useful voltage and by a second usefulcurrent driven by the second useful voltage.
 2. A field-device formeasuring and/or monitoring at least one specified physical and/orchemical parameter of a medium, said field device comprising: aphysical-to-electrical, measurement transducer issuing, at least attimes, at least one measurement signal corresponding with saidparameter; and field-device electronics as claimed in claim 1, saidfield-device electronics being electrically coupled with saidmeasurement transducer.
 3. The field-device electronics as claimed inclaim 1, wherein: said first voltage controller controls said firstinternal, useful voltage to be constant at a predetermined, firstvoltage level.
 4. The field-device electronics as claimed in claim 1,wherein: said second useful voltage is controlled as a function of aninstantaneous voltage level of said internal input voltage of thefield-device electronics.
 5. The field-device electronics as claimed inclaim 1, wherein: said second useful voltage is controlled as a functionof an instantaneous voltage level of a terminal voltage falling from theinput across the field-device electronics and derived from said supplyvoltage.
 6. The field-device electronics as claimed in claim 1, wherein:said second useful voltage is controlled as a function of aninstantaneous current strength of at least one of said three currentcomponents.
 7. The field-device electronics as claimed in claim 1,wherein: said second useful voltage is controlled as a function of aninstantaneous current strength of said third current component.
 8. Thefield-device electronics as claimed in claim 1, wherein: said seconduseful voltage is controlled as a function of the instantaneous currentstrength of said second current component and an instantaneous voltagelevel of said internal input voltage of the field-device electronics. 9.The field-device electronics as claimed in claim 1, wherein: saidfeeding, external energy supply provides a supply voltage with variablevoltage level.
 10. The field-device electronics as claimed in claim 1,wherein: said supply voltage delivered by said external energy supplydrives a supply current of variable current strength.
 11. Thefield-device electronics as claimed in claim 1, wherein: there isprovided in said operating and evaluating circuit at least onemicroprocessor, for which the first useful voltage, or a secondaryvoltage derived therefrom, serves, at least partially, as an operatingvoltage.
 12. The field-device electronics as claimed in claim 1,wherein: there is provided in said operating and evaluating circuit atleast one amplifier, for which at least one of the two useful voltages,or a secondary voltage derived therefrom, serves, at least partially, asan operating voltage.
 13. The field-device electronics as claimed inclaim 1, wherein: there is provided in said operating and evaluatingcircuit at least one Analog-to-Digital-converter, for which the firstuseful voltage, or a secondary voltage derived therefrom, serves, atleast partially, as an operating voltage.
 14. The field-deviceelectronics as claimed in claim 1, wherein: there is provided in saidoperating and evaluating circuit at least oneDigital-to-Analog-converter, for which at least one of the two usefulvoltages, or a secondary voltage derived therefrom, serves, at leastpartially, as an operating voltage.
 15. The field-device electronics asclaimed in claim 1, further comprising: means for comparing electricvoltages falling in the field-device electronics and/or electriccurrents flowing in the field-device electronics.
 16. The field-deviceelectronics as claimed in claim 1, wherein: the field-device electronicsproduces an alarm signal signaling the under-supplying of thefield-device electronics, at least when said operating and evaluatingcircuit detects a subceeding by the second useful voltage of a minimumuseful voltage limit value predetermined for the second useful voltageand a subceeding by the third current component of a minimum currentcomponent limit value predetermined for the third current component. 17.A method for operating a field-device electronics, which comprise aninternal operating and evaluating circuit for controlling the fielddevice, and an internal supply circuit for feeding said internaloperating and evaluating circuit, said internal supply circuit includinga first voltage controller, a second voltage controller, and a voltagestabilizer, said method comprising steps of: using a pair of electriclines for electrical connecting said field-device electronics with anelectrical energy supply being arranged external said field device andproviding a supply voltage; flowing through said pair of electric linesand through said field-device electronics a supply current driven bysaid supply voltage; flowing through said first voltage controller afirst current component of said supply current for providing in thefield-device electronics a first internal, useful voltage beingessentially constant; flowing through said second voltage controller asecond current component of said supply current for providing in thefield-device electronics a second internal, useful voltage variable overa predeterminable voltage range; and flowing through said operating andevaluating circuit both, a first useful current driven by the firstuseful voltage and a second useful current driven by the second usefulvoltage.
 18. The method as claimed in claim 17, further comprising astep of: deriving from the supply voltage an internal input voltage forthe field-device electronics and providing said internal input voltageto said internal supply.
 19. The method as claimed in claim 18, furthercomprising a step of: flowing through said voltage stabilizer a thirdcurrent component of the supply current for adjusting and maintainingsaid internal input voltage of the field-device electronics at apredeterminable voltage level.
 20. The method as claimed in claim 17,further comprising a step of: using said field-device electronics forvarying said supply current.
 21. The method as claimed in claim 17,wherein: the field-device electronics further comprises an electriccurrent controller for adjusting and/or modulating said supply current.22. The method as claimed in claim 20, further comprising a step of:flowing said supply current through said current controller.
 23. Themethod as claimed in claim 22, further comprising a step of: using saidelectric current controller for adjusting said supply current.
 24. Themethod as claimed in claim 20, further comprising a step of: using saidelectric current controller for modulating said supply current.
 25. Themethod as claimed in claim 20, further comprising a step of: using saidelectric current controller for varying said supply current.
 26. Themethod as claimed in claim 17, wherein: the internal operating andevaluating circuit includes an evaluation unit for providing ameasurement value representing a physical parameter of a medium conveyedin a pipe line and/or in a container, and the internal operating andevaluating circuit includes a driver unit electrical coupled with aphysical-to-electrical measurement transducer.
 27. The method as claimedin claim 26, further comprising a step of: flowing said first usefulcurrent through said evaluation unit.
 28. The method as claimed in claim27, further comprising a step of: flowing said second useful currentthrough said driver unit.
 29. The method as claimed in claim 26,wherein: the field-device electronics further comprises an electriccurrent controller for adjusting and/or modulating said supply current,said electric current controller being coupled with said evaluatingcircuit.
 30. The method as claimed in claim 29, further comprising astep of: using said measurement value for controlling said currentcontroller to adjust and/or to modulate said supply current.